Optical transmitters in some applications may comprise semiconductor lasers, such as Fabry-Perot (FP) lasers or distributed feedback (DFB) lasers. A FP laser comprises a semiconductor gain block (laser chip) that has two reflection facets at its opposite sides, e.g. in the direction of transmission. The semiconductor gain block between the two reflection facets may determine the cavity of the FP laser. The output wavelength of the FP laser is defined by the length of the cavity and the index of refraction of the gain block. The lasing condition of the FP laser may be satisfied for a plurality of wavelengths that correspond to a plurality of longitudinal modes in the cavity. The FP laser may have a relatively broad output wavelength spectrum, e.g. from about a few nanometers (nm) to about ten nm, around the gain peak. In fiber optic communication networks, such wavelength spectrum may not be suited for long transmission distances (e.g. greater than about ten kilometers (km)) and/or high data rate transmissions (e.g. greater than about one gigabits per second (Gbps)) because of chromatic dispersion in fibers.
A single mode laser, such as a DFB laser, may be more suited for long transmission distances and/or high data rate transmissions because of higher tolerance of its single mode operation (single output wavelength) to fiber dispersion. Similar to the FP laser, the DFB may comprise a laser chip with two facets. However, the lasing wavelength of the DFB laser is defined by a Bragg grating in the laser chip and the two facets may be coated with anti-reflection (AR) thin films to suppress multiple lasing modes. Typically, in wavelength division multiplexing (WDM) networks, multiple DFB lasers are used to emit a plurality of distinct wavelength channels. For example, the wavelength channels are spaced by about 50 GHz or about 100 GHz, which matches the wavelength grids specified by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T). However, to support WDM networks operations and provide additional spare parts, a large inventory of network cards (transponder cards) that comprise different DFB lasers is needed, which increases cost in WDM networks. Alternatively, fixed wavelength DFB lasers can be replaced by a smaller number of tunable lasers to reduce inventory size. However, since tunable lasers have a higher manufacturing cost than the DFB lasers, the overall cost of the system may not be substantially reduced.